The invention relates to a method for regulating the output of a steam or hot water boiler burning powdered fuel maintained in suspension, and to an apparatus for carrying out such method.
The regulation of the output of combustion chambers or fire boxes of steam and hot water boilers burning a fluidized powdered fuel, such boilers being provided with a heat transmitting surface in the fuel layer, is a rather complicated problem. These fire boxes cannot at present be adjusted to conditions of operation in case of a change of the quality of the fuel, such as its caloric value, ash content and particle size. Under certain conditions these fire boxes become either overcooled or insufficiently cooled, conditions which usually are the immediate causes of failures of operation of the boiler.
The overall amount of heat transmitted from the fluid layer to the heat transmitting surface located in the fluid layer is determined by the fundamental functional relation Q .times. k.multidot.F.multidot..DELTA.t, where Q is the amount of heat transmitted within a time unit in W, k is the heat transmission coefficient W/m.sup.2 .degree.C, F is the size of the surface of heat transmitting surfaces in m.sup.2, and .DELTA.t is the heat drop between the temperature of the fluid layer and the temperature of the medium passing through the heat transmitting system.
There are, for instance, known systems for regulating the output by eliminating a part of the heat transmitting system by screening a part of the grate surface in that this system is embedded in a stable layer instead of in the fluid layer, that is, the value of F of the fundamental relation is regulated.
Another method is also known, wherein the dependence of the variation of the magnitude of the coefficient k on the speed of fluidization is utilized. The fluid mass being burned is composed of two layers capable of being independently fluidized, only the lower layer being cooled.
Neither of these methods is suitable for fire boxes of large dimensions, and in addition they do not respond quickly enough to variations in the coefficient k due to variations of the amount of ash in the powdered fuel and to variations of its particle size.